Keyword Cipher

A keyword cipher is a form of monoalphabetic substitution. A keyword is used as the key, and it determines the letter matchings of the cipher alphabet to the plain alphabet. Repeats of letters in the word are removed, then the cipher alphabet is generated with the keyword matching to A,B,C etc. until the keyword is used up, whereupon the rest of the ciphertext letters are used in alphabetical order, excluding those already used in the key Worked Example.

Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Encrypted: K R Y P T O S A B C D E F G H I J L M N Q U V W X Z

With KRYPTOS as the keyword, all As become Ks, all Bs become Rs and so on. Encrypting the message "knowledge is power" using the keyword "kryptos":

Plaintext: K N O W L E D G E I S P O W E R Encoded: D G H V E T P S T B M I H V T L

Only one alphabet is used here, so the cipher is monoalphabetic.

The best ways to attack a keyword cipher without knowing the keyword are through known-plaintext attack, frequency analysis and discovery of the keyword (often a cryptanalist will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.

Classical cryptography Ciphers ADFGVX Affine Alberti Atbash Autokey Bifid Book Caesar Chaocipher Dvorak Four-square Great Hill Kama Sutra Keyword Nihilist One-time pad Permutation Pigpen Playfair Polyalphabetic Polybius Rail Fence Rasterschlüssel 44 Reihenschieber Reservehandverfahren ROT13 Running key Scytale Smithy code Solitaire Straddling checkerboard Substitution Tap code Transposition Trifid Two-square VIC cipher Vigenère Cryptanalysis Frequency analysis Index of coincidence Kasiski examination Miscellaneous Cryptogram Bacon Scytale Straddling checkerboard Tabula recta Cryptography History of cryptography Cryptanalysis Cryptography portal Outline of cryptography Symmetric-key algorithm Block cipher Stream cipher Public-key cryptography Cryptographic hash function Message authentication code Random numbers Steganography